(1) Field of the Invention
The present invention generally relates to an apparatus for controlling a heater for heating an oxygen sensor used in an internal combustion engine for measuring an air-fuel ratio in an exhaust gas.
(2) Description of the Prior Art
Recently, various control devices have been developed which are directed to an improvement in output power of an internal combustion engine, a reduction of fuel consumption or clarifying exhaust gas. Such control devices employ oxygen sensors without exception. As is well known, an oxygen sensor is used for measuring the concentration of an oxygen component contained in the exhaust gas. An oxygen sensor has a sensor element (sense portion) formed of a solid electrolyte or a semiconductor. An output signal of the oxygen sensor depends on the temperature of the sensor element thereof.
It is known that an oxygen sensor having a sensor element made of titania (TiO.sub.2) has an air-fuel ratio (A/F) characteristic as a temperature function of the sensor element as shown in the graph of FIG. 1. The vertical axis of the graph represents the air-fuel ratio, and the horizontal axis thereof represents the temperature of the sensor element. A stoichiometric air-fuel ratio exists between air-fuel ratios a.sub.1 and a.sub.2. When the actual air-fuel ratio is equal to or smaller than the air-fuel ratio a.sub.1 (a rich air-fuel ratio), a large amount of hydro carbon (HC) is contained in the exhaust gas. In contrast, when the actual air-fuel ratio is equal to or larger than the air-fuel ratio a.sub.2 (a lean air-fuel ratio), a large amount of nitric oxide (NO.sub.x) is contained in the exhaust gas. It can be seen from the graph of FIG. 1 that the temperature of the sensor element must be regulated so that it is maintained within the narrow temperature range between T1 and T2 so that the air-fuel ratio of the titania oxygen sensor can be kept within the narrow range between a.sub.1 and a.sub.2 including the stoichiometric air-fuel ratio.
From this viewpoint, a conventional oxygen sensor is equipped with a heater, which is subjected to a power supply control so that the value of resistance of the heater becomes equal to a definite resistance value. When the resistance value of the heater is regulated at the definite resistance value, the temperature of the sensor element is also regulated at a constant temperature. Such a power supply control is disclosed in Japanese Laid-Open Pat. Application Nos. 57-197459, 60-164241 or 60-202348, for example.
However, the aforementioned conventional temperature control of the oxygen sensor has the disadvantages described below. The temperature of the oxygen sensor is regulated due to the fact that the resistance value of the heater incorporated therein varies in accordance with a change of the temperature of the heater. It has been found from the inventors' careful study that it is difficult to regulate the temperature of the sensor element immediately after current supply to the heater is started by only controlling the resistance value of the heater so that it becomes equal to the definite value.
The above-mentioned problem arises from the structure of the heater. The resistance of the heater equals the sum of a resistance of a leading end portion thereof arranged in the vicinity of the sensor element and a resistance of a rear end portion thereof which is located close to a mounting portion of the oxygen sensor through which the oxygen sensor is fastened to an exhaust gas passage. In addition, the temperature of the heater leading end portion increases faster than that of the heater rear end portion due to different thermal capacities thereof. Thus, the heater resistance value obtained immediately after the current supply to the heater is actually started, shows a temperature lower than the temperature of the heater leading end portion. When the power supply to the heater is regulated so as to be equal to the definite value (target resistance value) for a while after the power supply to the heater is started, the temperature of the sensor element of the oxygen sensor increases over a target temperature (normal working temperature), or in other words, overshoots. The occurrence of overshooting the target temperature of the sensor element deteriorates precision in measuring the air-fuel ratio.